Splitters are used, for example, in connection with telephone lines over which, in addition to the conventional speech channel or ISDN signal (“Integrated Services Digital Network”), a high-rate digital signal is transmitted, such as an ADSL or VDSL signal (“Asymmetric Digital Subscriber Line” or “Very High Bit Rate Digital Subscriber Line”). The splitter thereby has the object of combining on the transmission side the two signals which are to be transmitted in different frequency bands for common transmission over the telephone line, and separating them again on the reception side, whereby in particular transmission takes place simultaneously over the telephone line in both directions.
FIG. 3 represents, by way of example, the use of a splitter in an ISDN/ADSL transmission system. In this context, FIG. 3 represents two subscribers who are in communication via bidirectional telephone lines 18 with an exchange 20. It is assumed from this that the upper subscriber has only an ISDN connection, and accordingly an ISDN transmitter 1 and an ISDN receiver 11 are allocated to him. The upper subscriber can therefore connect a suitable ISDN peripheral device directly to the telephone line 18, which has been connected to the corresponding ISDN connection of the exchange 20. An ISDN transmitter 1 and an ISDN receiver 11 are also allocated to the ISDN connection of the exchange 20. The lower subscriber, by contrast, has both an ISDN connection with an ISDN transmitter 1 and an ISDN receiver 11, and an ISDN connection 18 to an ADSL transmitter 2 and an ADSL receiver 12. In order for the ISDN signal and the ADSL signal to be combined for a joint transmission over the telephone line 18 on the transmission side, and in order for them to be separated again on the reception side, the use of a splitter 19 is necessary both at the subscriber and at the exchange 20.
FIG. 4 shows a diagrammatic representation of frequency bands, such as are found in an ISDN/ADSL transmission system of the type shown in FIG. 3. The frequency band of the ISDN signal is designated by A. For the ADSL transmission system, non-overlapping frequency bands are assumed, i.e. it is assumed that frequency band B in the upstream transmission direction (from the subscriber to the exchange) and the frequency band C in the downstream transmission direction (from the exchange to the subscriber) do not overlap. The frequency spectra of the ISDN signals, by contrast, are located in the same frequency band A for upstream and downstream transmission directions.
FIG. 5 represents, by way of example, a splitter 19 in accordance with the prior art, in a transmission system with a two-wire line 18, in which transmission takes place in both directions, i.e. bidirectional. The separation of the outgoing and return channel is effected with the aid of hybrid circuits 5, 6, which are also designated as a two-wire/four-wire or four-wire/two-wire conversion respectively. The hybrid circuits 5, 6 are coupled to the splitter 19 by means of transmitters 16, 17, whereby the hybrid circuit 5 is allocated to an ISDN circuit with an ISDN transmitter 1 and an ISDN receiver 11, while the hybrid circuit 6 is allocated to an ADSL connection with an ADSL transmitter 2 and an ADSL receiver 12.
The ISDN and ADSL signals received via the bidirectional transmission line 18 from the splitter 19 in different frequency bands are separated from one another, whereby the ISDN signal is conducted via the hybrid circuit 5 to the ISDN receiver 11. The ADSL signal is conducted via the hybrid circuit 6 to the ADSL receiver 12. Conversely, the splitter 19 also serves to combine the signals from the ISDN transmitter 1 via the hybrid circuit 5 and from the ADSL transmitter 2 respectively via the hybrid circuit 6 and to transmit them in the different frequency bands via the transmission line 18 to a corresponding receiver arrangement which can be designed, for example, as analogous to the circuit arrangement shown in FIG. 5.
To combine and separate the ISDN and ADSL signals respectively, the splitter 19 usually comprises suitable high-pass and low-pass structures. With the example shown in FIG. 5, the splitter represents in particular a purely passive circuit arrangement in the form of a three-gate array, which must be dimensioned in such a way that, on the one hand, the different systems do not incur mutual interference and, on the other, the impedance conditions are not too substantially altered. This represents a relatively large expenditure of effort, since because of the frequency bands being, as a rule, located close to one another, filters with steep damping flanks are required, and only purely passive components (resistors, inductors, and capacitors) can be used for the realisation of the arrangement.